Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface, showing the new design; and,

FIG. 2 is a front view of the display screen or portion thereof with a second embodiment of the graphical user interface.

The outermost dashed broken lines illustrate the environment of the design and form no part of the claimed design.

The de-emphasized half-tone portions illustrate the environment of the design or portions of the graphical user interface, and form no part of the claimed design. The dot-dash broken lines in the figures show boundaries that form no part of the claimed design. The areas within the dot-bash broken lines show portions of the graphical user interface that form part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 